The proposed research will examine the effects of bilateral lesions of the superior colliculus in rhesus monkeys on three psychological functions involved in visual information processing: detection, attention, and foveation. Monkeys will be trained and tested in a two-choice visual discrimination. Trials in which no discriminative cues are presented will be interspersed with cued trials; subjects will be reinforced for a press on a no-see bar during non-cued trials. The effect of superior colliculus lesions on the ability to detect stimuli will be determined by the frequency of presses on the no-see bar when cues are actually presented. Stimuli will be presented either too quickly for fixation to occur, or for a duration long enough to allow fixation. The effect of optic tectum lesions on the relationship between performance with and without fixation will reveal the involvement or lack of involvement of that structure in foveation ability. Discriminative stimuli will appear at the sites of the choice response or separated from the response sites; the ability to perform the task with separated cues measures the ability to shift attention from the response sites to the distant cues. The effects of superior colliculus lesions on visual discrimination with separated cues will indicate the role, if any, of the optic tectum in shifts of attention. This experiment may help elucidate the function or functions of the monkey superior colliculus. It may be possible to conclude from this study that the optic tectum is involved in several functions or is actually more than one structure, assigned a single name by tradition.